Since α-sulfo fatty acid alkyl ester salts (to be simply referred to as “α-SF salts”) demonstrate high cleaning strength, have favorable biodegradability and have little impact on the environment, their performance as detergent materials is evaluated highly. In addition, they are also advantageous in terms of costs in recyclable natural raw material systems in terms of resources as well, and are viewed with importance in terms of protecting the global environment as well.
A typical example of a method for producing α-SF salts consists of sulfonating a fatty acid alkyl ester using SO3 gas and the like to obtain an α-sulfo fatty acid alkyl ester (to be referred to as “α-SF”), followed by neutralizing with base to obtain an α-SF-containing aqueous solution in the form of a liquid or paste.
However, α-SF-containing aqueous solutions as described above increase in viscosity as the α-SF salt concentration increases resulting in the formation of a gel that lacks fluidity. When in a state such as this that lacks fluidity, transport by truck or ship becomes difficult, while dilute α-SF-containing aqueous solutions that are able to maintain fluidity result in the incurrence of transportation costs. Consequently, α-SF salts are typically distributed in solid form such as flakes or pellets.
In the production of granulated detergents, a solid α-SF salt as described above is dissolved in water to obtain an aqueous α-SF salt solution, and the aqueous α-SF salt solution is spray-dried with a detergent slurry composed of powdered raw materials, or powdered raw materials are granulated by using the aqueous α-SF salt solution as a binder. Consequently, aqueous α-SF salt solutions preferably have as high an α-SF salt concentration as possible while still maintaining fluidity. On the other hand, if the α-SF salt concentration becomes excessively high, the aqueous α-SF salt solution gels during production, which can cause blockage of flow paths of production equipment or adhere to heat-conducting surfaces causing a decrease in heat transfer efficiency. Methods have been previously reported that attempt to decrease the viscosity of aqueous α-SF salt solutions in order to resolve this problem.
For example, Patent Document 1 describes that the fluidity of an aqueous solution of an α-SF salt can be improved by blending specific amounts of an α-SF salt, an α-sulfo fatty acid disalt and an inorganic sulfate. Patent Document 2 describes a method for improving the fluidity of a surfactant slurry by incrementally concentrating a surfactant slurry containing an α-SF salt in the presence of sodium sulfate. Patent Document 3 describes that an aqueous α-SF salt solution facilitating easier handing by having an α-SF salt concentration of 30 to 60% by weight is obtained by using an inorganic sulfate as an essential component thereof. Patent Document 4 describes that an aqueous α-SF salt solution having favorable fluidity and an α-SF salt concentration of 40% by weight or more is obtained by containing an inorganic chloride. Patent Document 5 describes that an aqueous α-SF salt solution having an α-SF salt concentration of 30 to 60% by weight that facilitates easier handling is obtained by containing an inorganic sulfate and polyoxyalkylene-added polyvalent alcohol.